In the pulp and paper industry, various processes are employed for bleaching pulps. Bleaching is a continuation of the cooking process in which the ligneous material and coloring matter remaining in the chemical pulp are removed selectively with as little degradation of the pulp fibers as possible. Bleaching of pulp has advanced to a high degree of sophistication involving single stage and multi-stage procedures. The choice of bleaching agent has traditionally been dependent on whether the pulp is a mechanical pulp or a chemical pulp.
Mechanical and groundwood pulps have many desirable characteristics for low-cost papers, e.g. high yield, good bulk, high opacity, and good printing properties. The natural brightness of these pulps is, however, too low for the better grades of groundwood content papers. Also, the brightness of unbleached groundwood pulp varies with the species, the wood process, its age and its quality. Hence, the need for suitable bleaching processes arose in order to compensate for natural brightness variations of the wood; to obtain still brighter pulp to meet the ever-increasing demands for higher quality groundwood papers; and to improve the brightness of the pulp. The most important characteristic of mechanical pulp is the low cost of manufacture and this is due to its high unbleached yield. Oxidizing agents, for example, hydrogen peroxide (P) and hypochlorites (H) and reducing agents, for example, hydrosulfites (Hs), borohydrides (B), amino-boranes, peracetic acid, and bisulfites have been used since they raise the brightness of mechanical pulps without materially effecting the yield. Bleaching efficiency can sometimes be improved when combining these chemicals in multistage systems, e.g. two stages P-Hs or Hs-P or three stage Hs-P-Hs systems.
The bleaching of chemical pulp is accomplished in several stages. A "stage" constitutes a phase starting with addition and reaction of a chemical with a pulp, and ending with the washing of the pulp. Within each stage there are many process variables which are dictated by the type of reaction desired in that particular stage, and the operating conditions of the stage. These variables include: percent of chemical added and consumed, chemical concentration, consistency, temperature, time and pH. A series of such stages is called a bleaching sequence.
In such multistage bleaching processes, the first operation for the removal of lignin and other encrustants (usually following the digestion stage and the subsequent washing stage in a kraft, soda or sulfite process) consists in treating the pulp aqueous suspension with elemental chlorine in aqueous solution. A bleaching stage performed using elemental chlorine is designated as a C stage. Chlorination of the unbleached pulp so changes the ligneous impurities that they become in part soluble in water while, of the portions not readily soluble in water, a part is soluble in alkaline solutions such as dilute sodium hydroxide.
Among the agents used to bleach chemical pulp are elemental chlorine, chlorine dioxide, hypochlorites, chlorites, peroxides, chlorates, bichromates, and permanganates, as well as reducing agents, e.g. sulfurous acid, bisulfites, dithionites, and borohydrides. However, for chemical pulps (e.g. those made by the kraft, sulfite or soda processes) the bleaching agents traditionally used are chlorine and chlorine dioxide (a bleaching stage performed using chlorine dioxide is designated as a D stage) usually used in a multistage process. The first step thus usually consists of treating the pulp in an aqueous suspension with chlorine in solution in the aqueous phase. Chlorine dioxide may be used either in admixture with the chlorine, or in replacement of the chlorine, in this first stage. In some mills, hydrogen peroxide is used as a final bleaching treatment in a high density storage chest.
The next stages usually consist of a washing stage, preferably an alkaline washing or extraction stage (designated as an E stage). The products of pulp chlorination and of the oxidative bleaching stages are more soluble in an alkaline medium than in water, and they are generally taken out of the system by an alkaline extraction. For the attainment of brightness with strength preservation, for brightness stability and bleaching economy, the reaction products resulting from chlorination and oxidative bleaching are removed as they are formed in those operations by means of alkaline extraction. Caustic soda is the preferred agent, but other alkalis have been used. Some examples of multistage bleaching sequences include CEH, CEDP, CEDEDP and CEHH.
After a single or a multistage bleaching, it is common practice to treat or "sour" the bleached pulp with sulfur dioxide. This SO.sub.2 treatment serves many purposes.
For example, in the case of a multistage bleaching performed using sodium hypochlorite or calcium hypochlorite, hydrogen peroxide and chlorine dioxide among others, the SO.sub.2 treatment destroys the active bleaching agents remaining from the hypochlorite, peroxide and chlorine dioxide stages. Also, the SO.sub.2 treatment is required to bring the pulp solution to non-alkaline conditions in order to avoid undesirable losses with regard to brightness. It is well known that alkaline conditions tend to darken the pulp.
Presently, the pulp and paper industry universally employs SO.sub.2 dissolved in water to perform acidification or neutralization in both chemical and mechanical pulp bleaching. However, the use of sulfur dioxide has the consequence of destroying all the active bleaching agent residues remaining from bleaching stages. In the case of a bleaching stage performed with chlorine dioxide, it is desirable to treat the bleaching waters with SO.sub.2 to destroy the remaining bleaching agents.
However, when it comes to bleaching agents such as sodium or calcium hypochlorite and hydrogen peroxide, their destruction is by no means necessary because these compounds present low toxicities and do not interfere with subsequent paper making operations. As a matter of fact, the presence of such residuals would be extremely desirable because it would allow the bleaching process of the pulp to slowly continue during subsequent pulp and paper making operations. This prolongation of the bleaching process would likely yield a final product possessing enhanced brightness properties without increasing operational costs. It is also to be noted that these bleaching agents and their residuals also possess mild biocide properties that allow for the prevention of bacterial growth in the pulp solution. The proliferation of bacteria in the pulp solution usually leads to undesirable darkening of the pulp itself.
Furthermore, the use of aqueous SO.sub.2 solutions may present serious health risks for the workers handling the treated pulp as well as potential environmental hazards that could be caused by the disposal of corrosive and polluted SO.sub.2 solutions in natural effluents.
Therefore, a souring agent possessing the property to diminish alkaline conditions while maintaining desirable bleaching agent residuals in the pulp solution would be highly desirable.